Short Course Radiation Therapy and Combination Chemotherapy for the Treatment of Stage II-III Rectal Cancer

This phase I trial investigates how well short-course radiation therapy followed by combination chemotherapy works in treating patients with stage II-III rectal cancer. Radiation therapy uses high energy x-rays to kill tumor cells and shrink tumors. Chemotherapy drugs, such as leucovorin, fluorouracil, oxaliplatin, and capecitabine, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Giving short-course radiation therapy and combination chemotherapy may reduce the need for surgery and therefore improve quality of life.

Inclusion Criteria: * Histologically confirmed rectal adenocarcinoma * Patients must have stage II (cT3, cN0) or stage III (cT1-3, cN1-3) tumor as staged by MRI * No evidence of metastatic disease * Resectable primary lesion * Karnofsky performance status (KPS) \>= 70 or Eastern Cooperative Oncology Group (ECOG) 0-2 * Absolute neutrophil count (ANC) \> 1.5 cell/mm\^3 * Hemoglobin (Hgb) \> 8.0 gm/dL * Platelets (PLT) \> 150,000/mm\^3 * Total bilirubin \< or equal to 1.5 x upper limit of normal * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) \< or equal to three times upper limit of normal * If a woman is of childbearing potential, a negative serum pregnancy test must be documented prior to initiation of radiation therapy Exclusion Criteria: * Active treatment of a separate malignancy * Distant metastatic disease as assessed by staging positron emission tomography (PET)/computed tomography (CT) or CT of the chest and abdomen within 6 weeks of starting radiation therapy * Prior radiotherapy to the region of the study cancer that would result in overlap of radiation therapy fields * Pregnant and/or breastfeeding * Medical/psychological contraindication to MRI